This invention relates generally to an earthquake resistant reinforced wall structure for concrete and masonry structures. More particularly, this invention relates to a construction approach utilizing a reinforcing framework to improve the load bearing characteristics, especially of dynamic loads, of walls, columns and other structural elements built in accordance with the present invention.
The construction industry has repeatedly tried to address a multitude of long-standing problems, foremost among these problems are concerns directed to the achievement of a wall construction having structural strength sufficient to resist earthquakes, and concerns directed towards saving material costs and assembly time during construction. Another pertinent long standing problem in the construction field is the general inability to provide extensive access within walls for the use of concrete compacting equipment therein. A closer look at these long-standing problems will allow one to fully appreciate the solutions offered by the invention to be described herein.
Building codes, which govern construction, require reinforcement in structures made of concrete and masonry materials such as shear walls, columns and beams, so that these structures will ". . . carry all factored gravity loads . . . including tributary loads and self weight, as well as the vertical force required to resist overturning moment calculated from factored forces related to earthquake effect" (as stated in an American Concrete Institute Committee report).
The construction field currently employs practices that have evolved during a long-standing preoccupation with the aspect of achieving an acceptable degree of structural strength in free-standing structures. Generally, when acted upon by external forces, such as earthquake and high velocity wind gusts, a free-standing structure will experience tensile strain on one side and compressive strain on the opposite side. This condition will alternate cyclically. For this reason, building codes require that vertical steel reinforcing bars be placed at both ends of each wall or other structure. Under compressive loading, these reinforcing bars tend to fail by buckling so that builders are required to further reinforce the reinforcing bars with lateral restraints such as hoops placed on prescribed minimum centers. Two common prior forms of transverse constraints are first, a steel rod spirally wound about the perimeter of the vertical reinforcing bars, and second, a series of vertically spaced transverse "hoop ties" connecting the reinforcing bars around their periphery. Disadvantageously, both of these prior transverse constraints must be custom formed either off site or at the construction site. The prior type of transverse constraints are then mounted into place by craftsmen and are manually tied to the vertical reinforcing bars with wires.
In prior construction methods, after a reinforcing framework is completed using prior transverse constraints, concrete is poured within boundary forms to complete the structure.
Using prior construction techniques, the resultant structure's performance under load is determined to a great extent by the quality of workmanship in forming and typing prior transverse constraints into place. Usually, weakness in a structure that is conventionally reinforced can be traced mainly to: 1) loosely fitting transverse elements and 2) voids in the concrete due to poor compaction. With regard to factor #2, another long-standing problem in construction is highlighted. Conventionally, vibratory compactors are utilized within free standing structure during the pouring and setting of concrete during construction. The compactors use vibrations to cause a wet concrete flow to achieve an even distribution of concrete. The compactors densify concrete, thereby eliminating air pockets and voids in the concrete before said concrete dries with void-related weak points therein. Unfortunately, current construction practices hinder the use of compactors by limiting access for the use thereof. This is due, in large part, by the accepted practice of using convention confinement "rebar" (short for reinforcing bar) ties having seismic hooks or other protrusions that hinder the insertion of vibratory compactors into a structure. Often, conventional reinforcing arrangements present obstacles that block free access for proper, extensive use of compactors. The resultant limitations upon effective compacting action causes an increase in the size and frequency of voids in the finished structure, thus reducing the strength and dynamic load carrying characteristics of the finished structure.
As mentioned previously, structures being subjected to an earthquake generally are subjected to cyclic loading. A problematic aspect of such cyclic loading is that extremely large vertical shear forces are generated near the ends of structural walls. These vertical shear forces disasterously tend to separate the ends of a structure from its more central portions because traditional prior lateral reinforcing spirals and hoops do not effectively transfer end loading stresses to central areas of a structure.
The construction industry both in the United States and abroad has recently begun to realize that conventional methods of reinforcing concrete shear walls, columns and beams, using rebar hoops with hooked ends, including typing procedures for columns, and other methods applicable to shear wall construction are inadequate to provide structural stability during earthquakes. A higher precision in the positioning of boundary reinforcing bars to uniformly resist compressive buckling is needed to address the problems currently being encountered in the construction industry. Current reinforcing methods do not provide adequate space in poured concrete for effective positioning and insertion of compacting equipment. The dilemma of improving the strength and placement precision of reinforcing steel while lowering construction costs is a long-standing problem in this field. It has been addressed, albeit inferiorly, by many prior art solutions and attempts. There exists a need, therefore, for an improved method of reinforcing free standing structures, especially walls, columns and beams, that will provide an improved fit over vertical reinforcing bars, take less space, thereby leaving more room for improved compactor access, and that will reduce both material costs and assembly time, thereby lessening installation expense.
An earthquake resistant structure is needed that utilizes an improved reinforcing framework that allows for the achievement of greater overall structure ductility and more predictable structure response to loading. Moreover, such an improved reinforcing framework is needed that will allow for close control of material and assembly tolerances, thereby allowing achievement of precision dimensions and precision positioning of lateral reinforcing elements. A needed construction improvement that would result in increased dimensional uniformity throughout the structure would facilitate the use of less concrete and smaller diameter rebars if such uniformity were achievable without a loss in structural strength. The present invention fulfills all of these needed construction improvements and provides further related advantages. The present invention advantageously greatly improves structure resistance to large dynamic loads, especially those encountered during earthquakes.